1. Technical Field
This invention relates to a hexagonal barium ferrite magnet, method for preparing the same, and a polar anisotropy ring magnet.
2. Background Art
Currently, hexagonal strontium and barium ferrites of the magnetoplumbite type (M type) are mainly used as oxide permanent magnet materials and they are manufactured into sintered magnets and bonded magnets. Strontium or barium ferrite sintered magnets are-prepared by blending and mixing Fe.sub.2 O.sub.3 and SrCO.sub.3 or BaCO.sub.3 source materials, calcining, pulverizing, compacting in a magnetic field, and firing. In the process, it is a common practice to add CaCO.sub.3 and SiO.sub.2 together as shown in Examples of JP-A 291901/1991 and 5802/1992, for example.
Hexagonal barium ferrite is advantageous in reducing manufacture cost because BaCO.sub.3 used as the source material is less expensive than SrCO.sub.3 for strontium ferrite. However, hexagonal barium ferrite sintered magnets are less likely to offer high coercivity (iHc) since their crystal magnetic anisotropy is about 10% less than that of strontium ferrite. Therefore, hexagonal strontium ferrite sintered magnets are used in products which require iHc of at least 3 kOe, for example.
On the other hand, when CaCO.sub.3 and SiO.sub.2 are added together during preparation of strontium or barium ferrite sintered magnets as in the prior art, the shrinkage factor during sintering is significantly different between c and a axis directions, with their ratio of c axis direction/a axis direction ranging from about 1.5 to about 2.1. As a result, shrinkage during sintering causes substantial deformation, which imposes the necessity to design the mold by previously taking into account a deformation amount or to carry out substantial post-working like polishing and machining, for example, resulting in an increase of manufacture cost.
A polar anisotropy ring magnet is known as a typical magnet that experiences substantial deformation due to such differential shrinkage factor. This magnet is of ring shape with its inner or outer circumferential surface being strongly magnetized in multiple poles and finds use in stepper motors or the like. In preparing the ring magnet, a ring-like molded body is formed while alternating poles are arranged around its circumferential surface to effect magnetic field orientation for orienting c axis in the direction of an arrow in FIG. 7(a), and it is then sintered. However, since the body has great anisotropy of shrinkage factor as mentioned above, the circumferential surface is deformed as shown by broken lines in FIG. 7(b). Ridges developed on the circumferential surface must be ground away and hence, a substantial grinding allowance must be furnished.